1. Technical Field
Exemplary aspects of the present disclosure generally relate to a transfer device and an image forming apparatus including the transfer device, and more particularly to an image forming apparatus such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
There is known an image forming apparatus equipped with a transfer device that transfers a toner image from an image bearer onto a recording medium interposed in a so-called transfer nip at which the image bearer and a nip forming device contact. A transfer bias is applied to the transfer nip by a transfer bias power source to transfer the toner image from the image bearer onto the recording medium.
As an example of such a transfer device, a secondary transfer device is known to transfer secondarily a composite toner image formed on an intermediate transfer belt onto the recording medium. Toner images formed on a plurality of photoconductors are transferred onto the intermediate transfer belt in primary transfer nips between the photoconductors and the intermediate transfer belt such that they are superimposed one atop the other, thereby forming the composite toner image. The composite toner image is then transferred from the intermediate transfer belt to the recording medium by the secondary transfer device.
The secondary transfer device includes a secondary transfer roller as a nip forming device and a secondary-transfer opposed roller. The secondary transfer roller contacts the intermediate transfer belt serving as an image bearer. The secondary-transfer opposed roller is disposed opposite the secondary transfer roller via the intermediate transfer belt and contacts the intermediate transfer belt from the back thereof. The intermediate transfer belt is interposed between the secondary transfer roller and the secondary-transfer opposed roller to form the transfer nip.
One of the secondary-transfer opposed roller and the secondary transfer roller is connected to the transfer bias power source while the other roller is grounded. The transfer bias power source applies the transfer bias including a direct current (DC) voltage to the one of the secondary-transfer opposed roller and the secondary transfer roller. With this configuration, a secondary transfer electric field is formed between the secondary-transfer opposed roller and the secondary transfer roller so that the toner image moves electrostatically from the secondary-transfer opposed roller side to the secondary transfer roller side. A recording medium is fed to the secondary transfer nip in appropriate timing such that the recording medium is aligned with the toner image formed on the intermediate transfer belt. Due to the secondary transfer electric field and a nip pressure in the secondary transfer nip, the toner image on the intermediate transfer belt is secondarily transferred onto the recording medium.
In this configuration, the transfer bias applied to the secondary transfer roller or to the secondary-transfer opposed roller is subjected to constant current control. When controlling the transfer bias under constant current control, the applied voltage changes in accordance with changes in electrical resistance of the intermediate transfer belt and the secondary transfer roller caused by environmental changes such as temperature and humidity change so that transferability is stable regardless of the environmental change.
In such a configuration described above, when using a recording medium having a coarse surface or an embossed surface such as Japanese paper (also known as Washi), a pattern of light and dark (unevenness of image density) according to the surface condition of the recording medium appears in an output image. More specifically, toner does not transfer well to such embossed surfaces, in particular, recessed portions of the surface. This inadequate transfer of the toner appears as a pattern of light and dark patches in the resulting output image.